The present invention relates to compositions comprising a catalyst for exothermic reactions conducted on a fixed bed and to a metal diluent used to reduce or eliminate the formation of hot spots in the fixed bed.
In particular, it relates to compositions in which the catalyst is a catalyst for the oxychlorination of ethylene to 1,2-dichloroethane.
Removal of the reaction heat in exothermic reactions by the cooling fluid is decisive for reaction control and therefore for the possibility to achieve high conversions and selectivities.
Whereas in fluid-bed operations this problem is scarcely important due to the high overall exchange coefficient that can be achieved in these conditions, in the fixed-bed technology the problem of removing heat is extremely important, since at the inlet of the bed the concentration of the reagents is high and therefore the reaction rate and the production of heat are the highest. The temperature inside the catalytic bed therefore tends to rise rapidly, creating regions of high temperature (hot spots) which produce considerable problems in terms of rapid aging of the catalyst and cause a consequent loss of selectivity due to the increase in secondary reactions. Bearing in mind that the amount of heat exchanged is governed, for a given cooling surface and for a given overall exchange coefficient, by the difference between the temperature inside the bed and the temperature of the cooling fluid, and that in normal conditions the rate of heat exchange is regulated by said temperature difference, the temperature in the hot spot will tend to rise until the difference in temperature removes all the heat produced by the reaction.
In the final part of the bed, instead, the reaction rate (and therefore the production of heat) is very low and hot spots accordingly do not occur.
In order to reduce the hot spot temperature by acting on the catalyst one can use two approaches:
using a scarcely active catalyst in the region of the catalytic bed at the inlet of the reagents;
diluting the catalyst in said region by using inert solid diluents.
The diluents used so far comprise materials such as graphite, silicon carbide, macroporous carbon, low surface area alumina, silica and glass beads.
These diluents, due to their low thermal conductivity coefficient, are not suitable to effectively transfer heat from the hot spot region to the wall of the heat exchanger.
Furthermore, again due to their low thermal conductivity, the diluents are unable to adequately transfer heat from the regions where, due to uneven mixing of the catalyst and of the diluent, peaks in the concentration of the catalyst occur, with consequent forming of hot spots.
It has now been unexpectedly found that the use as diluent of metals which are inert toward the reagents and the reaction products and having high thermal conductivity allows not only to improve the yield and selectivity of the catalyst and therefore the productivity of the plant but also to reduce or avoid the loss and/or aging of the catalyst in cases in which these problems tend to occur.
In particular, in the case of the oxychlorination of ethylene to 1,2-dichloroethane, diluents with high thermal conductivity allow to provide the reaction in a single stage instead of in multiple stages as normally occurs.
The diluents that can be used in the compositions according to the invention are metals with a thermal conductivity of more than 0.5 W/cm/K (value considered in the temperature range from 400K to 1573K, equal to 127xc2x0 to 1000xc2x0 C).
Copper has a thermal conductivity (W/cm/K) of 3.93 at 400K and 3.39 at 1573K; the values for aluminum is 2.4 at 400K and 2.18 at 800K; the values for nickel are 0.8 and 0.76 at 400K and at 1200K respectively; zinc has a conductivity of more than 1 in the temperature range being considered.
The following are examples of coefficients related to materials not included among the usable ones: 0.13 W/cm/K at 673K for alumina: 0.04 and 0.01 for graphite at 400K and 1200K; 0.19 and 0.25 for stainless steel at 573K and 973K.
The metals usable in the compositions according to the invention are chosen so as to be substantially inert with respect to the reagents and to the products of the reaction in which they are used.
Copper is the preferred metal, due to its high thermal conductivity and high density which allows to provide high heat capacity per unit volume of metal and therefore to absorb and then rapidly transfer considerable amounts of heat.
Aluminum and nickel, too, are conveniently usable, particularly in reaction conditions in which high chemical inertness is required.
The metallic diluents are preferably used with a geometric shape and dimensions which are similar to those of the granular catalyst with which they are mixed. It is also possible to use different shapes and dimensions.
Preferred shapes are those that provide a wide surface area per unit volume of diluent associated with significant void percentages. This is done in order to facilitate heat exchange and reduce pressure losses.
Examples of these shapes are the cylindrical ones with a through bore having a wide diameter and annular shapes.
Examples of cylindrical shapes are multilobed shapes with through bores at the various lobes and other shapes with a large geometric area.
Shapes of this type (described for catalysts and carriers) are reported in U.S. Pat. No. 5,330,958, whose description is included herein by reference.
The dimensions of the cylindrical shapes are generally between 3 and 10 mm in height and 5-10 mm in diameter.
The percentage of diluent is a function of the exothermic nature of the reaction and of its kinetics.
Percentages from 10 to 80% by volume on the mixture can be used conveniently.
The catalytic compositions that contain the metal diluent are used so as to form the bed in the part at the inlet of the reagents.
It is also possible to use various bed layers in which the concentration of the catalyst rises toward the lower part of the bed.
A typical example of exothermic reaction conducted on a fixed bed in which the compositions according to the invention can be used conveniently is the oxychlorination of ethylene to 1,2-dichloroethane.
Examples of other reactions are: oxidation of n-butane to maleic anhydride; oxidation of o-xylene or naphthalene to phthalic anhydride; synthetic natural gas from methane; vinyl acetate from ethylene and acetic acid; ethylene oxide from ethylene.
As mentioned, in the case of the oxychlorination reaction it has been found that, in addition to the advantage of higher yields and selectivities, the use of diluted catalysts according to the invention allows to conduct the reaction in a single stage instead of multiple stages, as normally occurs in the processes of the prior art.
The diluted catalysts according to the invention are used under the reaction conditions that are used normally; however, it is possible to optimize said conditions in order to utilize the higher performance of the catalysts in the best possible manner, in terms of both yield and selectivity.
Catalysts that can be diluted with the metal diluents comprise all the catalysts that can be used in exothermic reactions conducted on a fixed bed.
In the case of the catalysts for the oxychlorination of ethylene to 1,2-dichloroethane, the representative and preferred usable catalysts are based on cupric chloride or cuprous hydroxychloride, comprising promoters chosen among chlorides of alkal metals and/or chlorides of alkaline-earth metals, optionally of rare earths.
These catalysts are supported on inert porous supports, particularly alumina with a surface area between 50 and 300 m2/g.
Catalysts of this type are amply described in the literature and in particular in EP-A-176432, the description of which is included herein by reference. In the catalysts described in EP-A-176432, the concentration of cupric chloride is lower on the surface than inside the catalyst granule.
The following examples are provided to illustrate but not to limit the scope of the invention.